I. Field of the Invention
The present invention relates generally to forward power control in a code division multiple access ("CDMA") mobile communications system and particularly to power control using the forward channel in a CDMA system.
II. Description of the Related Art
CDMA modulation for utilizing the RF spectrum for mobile communications is one of several techniques in which a communication system serves a large number of users. CDMA technology improves the quality of service by providing robust operation in fading environments and transparent or "soft" handoffs. A soft handoff occurs when a mobile station establishes a connection to a target base station, while briefly maintaining the connection with a source base station and on occasion a third base station. As the mobile moves from its current cell (source cell) to the next cell (target cell), a traffic channel connection is simultaneously maintained with both cells.
A smooth soft handoff is accomplished by the aid of the mobile station. FIG. 1 illustrates the forward link in a soft handoff and FIG. 2 illustrates the reverse link. On the forward link, the mobile 10 uses a rake receiver (not shown) to demodulate two separate signals from two different base stations 12, 14. The two signals are combined to yield a composite signal of better quality. On the reverse link in FIG. 2, the mobile's 10 transmit is received by both base stations 12, 14. The two cells demodulate the signal separately and send the demodulated frames back to the base station controller 16. The base station controller ("BSC") 16 contains a selector 18 that selects the best frame out of the two that are sent back.
The CDMA system accomplishes smooth operation by strict power control. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users. The power of each individual user must be carefully controlled such that no one user is unnecessarily interfering with others who are sharing the same band. This is also true during soft handoffs, where the mobile station transmits on the reverse link a power control message to the base station. The IS-95 standard specified that the mobile must report back to the base station the quality, related to the signal-to-noise ratio ("SNR"), of the forward link. The mobile station gathers statistics on error performance on whether it is decoding the base station signals correctly and informs the base station via a power control message. The base station then adjusts its power to the specific user accordingly. Symbols transmitted on the forward and reverse channels are formatted into frames, each frame having a 20 millisecond length. U.S. Pat. No. 5,504, 773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION" issued Apr. 2, 1996 of Padovani et al., and assigned to the assignee of the present invention, recites a more detailed explanation of these frames.
The mobile station updates the power control on a frame by frame basis. The high update rate of power control messages enable the base stations to fine tune their transmit power to each individual mobile station, keeping their transmit power at the minimum level required to maintain a link, thus minimizing the total interference of the system and improving the capacity of the system. The frame by frame power control process is disclosed in U.S. Pat.No. 5,383,219, entitled "FAST FORWARD LINK POWER CONTROL IN A CODE DIVISION MULTIPLE ACCESS SYSTEM" issued to Wheatly HI, et al., and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference as if fully set forth herein.
A selector is responsible for base station processing requirements. The selector of the land network determines a traffic rate at which a frame is sent to a mobile station and sends the frame to all base stations communicating with that particular mobile. The number of bits per frame transmitted on the reverse channel varies according to the traffic rate. A more detailed description of the traffic rate will be provided below.
During a soft handoff, a plurality of base stations transmit the frame to the mobile. After combining data from possible multiple base stations, the mobile determines whether the last frame has been received and decoded correctly. If the mobile correctly decoded the last frame, the mobile sets the power control bit in the next frame that is transmitted to the base stations. The selector monitors and synchronizes communication when multiple base stations simultaneously communicate with a mobile station. Since the selector knows the rate at which the base stations transmitted the last frame to the mobile and now has feedback from the mobile on whether that frame was correctly decoded, the selector compiles a table of statistics on the error rates that the mobile station is incurring at each rate. As an example, for a given traffic rate, the selector calculates an error rate of "El."
The selector determines the power level or traffic gain at which the next frame is to be transmitted by comparing to zero the respective difference just calculated. For example, if the frame is to be transmitted at a full rate and E1&gt;0, the power level will be P.sub.nominal +P, where P is a function of the value of E1 and P.sub.nominal is the power level set by the carrier for that geographical area. If E1=0, the power level will be P.sub.nominal. If E1&lt;0, the power level is P.sub.nominal -P. The selector forwards the next frame to be transmitted to the mobile to the base stations that are communicating with the mobile. An indication of the power level at which the frame is to be transmitted is included with this frame.
The problem with the type of power control described above is that there is delay between the time the selector calculates the traffic gain, and when the traffic gain is transmitted to the base stations and programmed into the application specific integrated circuit ("ASIC"). Each base station and the selector run identical power control algorithms, but the result of the selector algorithm is delayed due to back haul.
During a soft handoff operation one of the base stations communicating with the mobile station may receive an erroneous SNR signal from the mobile, or may receive no command signal from the mobile station. Because power control signals on the forward channel are transmitted at a high rate, the base station may transmit at an undesirably high or low power level for several frames prior to receiving the most reliable traffic gain from the selector. If the base station transmits at a power level too high, the overall interference of the system is increased. If the base station transmits at a power level too low, then communication quality is degraded.